1. Field of the invention
The present invention relates to a positive photoresist resin useful for semiconductor devices and to a chemical amplified positive photoresist composition comprising the same.
2. Description of the Prior Art
It is known that positive photoresist compositions consist, in general, of two or three components. The two-component system photoresist composition comprises a polymeric resin which can be chemically changed by the reaction with acid and a compound which can generate acids when being exposed to light (hereinafter referred to as "photoacid generator") while the three-component system comprises a resin which can be dissolved in aqueous alkali solutions (hereinafter referred to as "alkali soluble resin"), a dissolution inhibitor and a photoacid generator.
As for the resin of the two-component system photoresist composition, it is selected from phenolic resins containing a poly(4-t-butoxycarbonyloxystyrene) chain protected by t-butoxycarbonyl groups and acrylic resins containing a poly(t-butylmethacrylate) protected by t-butyl groups. The phenolic resins require a strong acid to remove the protecting group from themselves. Thus, unless a baking process is executed immediately after an exposing process, a base present in the air causes neutralization in the exposed region, resulting in development into a T-type pattern. Meanwhile, the t-butyl group-protected acrylic resins are superior in adhesiveness but poor in etch resistance.
As an alternate for the photoresist resin was developed poly(t-butyl-4-vinylbenzoate). A significant disadvantage of this resin is that the resonance between the phenyl ring and the carbonyl group allows the resin to show a large absorption peak at around 248 nm, which belongs to a light source useful for microlithography. In order to overcome the disadvantage, study has recently been focused on copolymeric resins. For example, copolymers of para-hydroxystyrene and para-t-butoxycarboxystyrene, copolymers of para-hydroxystyrene and para-tetrahydropyranyloxystyrene, and copolymers of para-hydroxystyrene and t-butylacrylate are found to be reduced in absorption at around 248 nm. However, the photoresist compositions resulting from these resins still show a problem in that desired patterns cannot be obtained if the compositions are subjected to a baking process with the lapse of time after exposure to light.